The production of slit hollow profiles, so-called slit profiles, which are produced from an essentially flat blank using U-O-forming or using rolling-up technology, which is also known under the name InnForm T3® technology of the applicant, and are joined in a firmly bonded manner along the longitudinal slit, is known from the prior art. Rolling-up technology (InnForm T3) is taken to mean producing a hollow profile from a cut blank using a multi-stage press, single presses or press lines, for example as a follow-on composite, wherein the presses usually form the mostly flat cut blank by means of metal forming and bending operations with or without the use of a forming core. In the case of U-O-forming, usually an essentially flat cut blank is firstly formed into a U-shape using a corresponding punch and a lower die and is then formed into an O-shape using a second upper die. U-O-forming can also be carried out in press lines. However, when using both rolling-up technology and U-O-forming, springbacks and elastic and/or plastic distortions occur in the sheet plane, so that there is not the sufficient shape and dimensional stability required when it is joined to form a closed hollow profile. The springbacks and distortions of the blank material, which increasingly occur particularly in higher-strength and super high-strength steels, result in deviations from the desired contour occurring at the joint edge before welding. As a result, the slit of the hollow profile exhibits deviations from its desired width, for example, so that problems can occur when welding the slit hollow profiles.
It is known from the German patent specification DE 103 29 424 B4 to choose the cut blank when producing hollow profiles with a variable cross-section in the longitudinal direction in such a way that an excess of material or a shortage of material is provided in the transition areas, so that when it is formed into a hollow profile no material thickenings or attenuations occur at the transition areas. In contrast, the German patent specification DE 100 45 938 C1 relates to a method for producing closed hollow profiles, in which the edges of the cut blank which are opposite one another can be brought fully edge to edge, so that they can be welded easily without using filler materials. This is achieved by firstly bending or roll forming the cut blank in a forming-bending tool, so that the longitudinal edges of the blank are brought edge to edge. During this bending or roll-forming process, the sheet blank is compressed via an axial punch in the axial direction, so that the abutting edges are perfectly in contact edge to edge and a circumferential bulge is produced in the truncated cone sections of the hollow profile. However, this axial compression requires an additional axial punch and principally a further production step.
Taking this prior art as the starting point, the object forming the basis of the present invention is to provide a method for producing closed hollow profiles, by means of which method highly dimensionally stable, slit hollow profiles can be simply produced which can be easily joined to form closed hollow profiles.